Lasers that emit infrared light have a wide field of application such as environmental measurement. In many kind of lasers, quantum cascade lasers made of semiconductors are small sizes and highly convenient, and enable high-accuracy measurement.
The quantum cascade laser includes, for example, an active layer including quantum well layers in which GaInAs and AlInAs are alternately stacked. Further, the quantum cascade laser has a structure in which both side surfaces of the active layer are placed between, for example, InP cladding layers. In this case, the quantum well layers cascade-connected can emit infrared laser light with a wavelength of 4 to 10 μm by the intersubband optical transition of carriers.
However, in the case of infrared laser light with a wavelength of 10 μm or more, infrared laser light emitted from the active layer is absorbed in the InP cladding layers on both side surfaces of the active layer because of light absorption due to lattice vibrations (phonons) of InP. Consequently, light emitting efficiency is reduced, and it is difficult to obtain high-power laser light.